Power line transmissions are extremely difficult to monitor accurately because of the high voltages and currents flowing through the lines. Heretofore massive costly structures were needed to connect high voltage lines to ground to obtain accurate voltage measurements. These devices employ potential transformers (PT's) or coupling capacitor voltage transformers (CCVT's), which only measure voltage, mounted on a massive insulated standoff supported by a ground base. Just one of these systems can cost as much as $20,000 or more in 1986 dollars for a single two hundred kilovolt phase conductor.
Voltage in many power transmission lines is measured with a PT or CCVT while current measurement heretofore has required the use of a separate device, typically a current transformer (CT). Voltage and current transformers are used for revenue metering when accuracy better than 0.3% is desirable and for protective relaying where fast circuitry response and interference free data transmission are desirable. These PT/CCVT and CT transformers are massive in design and structure because they must be in direct contact with the high voltage line on the primary side, and with earth or ground potential on the secondary side where the voltage current measurement is made. The ground side of the PT and CT transformers typically uses ohms per centimeter to physically bridge the voltage gradient. These porcelain high resistance bushings in order to have sufficient insulation to bridge power company line to ground voltage gradients of up to roughly 500 kilovolts (KV) are tall, about 20 feet, and thus are concomittantly massive and costly. In addition, the accuracy of these devices is limited and not typically within the 0.3% desired.
Additional problems with recording the data from a PT/CCVT are that cables leading from these devices to metering or relaying equipment located elsewhere can sometimes pick up high frequency noise which can interfere with optimal operation of relay, control and metering equipment. These effects further increase costs and also reduce the efficiency of the systems.
Another disadvantage is the high cost of installing these transformers, and maintenance operations on them, which require costly high voltage power line outage time since the power transmission line must be disconnected from use while installation or maintenance proceeds. These transformers are also used to derive the phase angle of the power by processing the output of a PT transformer and separately a CT transformer, at separate processing equipment located some distance from the PT and CT. With such two linear outposts delivering data from two different and independent transformers being transmitted over a distance into a third processing station, accuracy of course is limited and the costs can be significant.
Another method of measuring transmission line voltage is by use of an electrostatic voltmeter, also called a capacitive voltmeter which is a non-contact instrument for measuring electric field strength. While these devices can be used to measure high voltages, such as 5-120 KV (kilovolts), they have never been widely accepted for use by the power industry due to the lack of accuracy and stability and the need to convert electric field strength to voltage.
Of two other devices for monitoring certain parameters of power transmission lines, one is in the shape of a toroid, and a second is in the shape of a shoebox. Both are freely mounted on and in contact with a transmission line. The toroid has circuitry which measures voltages and phase angle and both devices measure current and conductor surface and ambient temperature and transmit the information to a remote ground station via a radio data link. These systems are line powered and self-contained modules. Line temperature and line current are needed to calculate the dynamic thermal line rating. The sensing device mounted in the housing on the transmission line includes a radio frequency (RF) transmitter transmitting measured information to a ground station where it is then processed. The shoebox device is not without problems since it does not measure voltage and thus, cannot measure phase angle. The toroidal device is also not without problems because the voltage sensor employing an electrostatic voltmeter, rather than a more accurate PT or CCVT, has accuracy typically of only about plus or minus 5% at best. Such accuracy is typically not adequate for most power company revenue metering and protection and load management or networking purposes.
An additional problem with these systems is the use of radio frequency data links as the sole means to transmit data from the power line down to ground station receivers which use radio frequencies requiring a Federal Communications Commission (FCC) license. Obtaining such a license can be difficult for power companies operating in urban areas already crowded with radio transmissions which may need, for example, twenty or more transmitters, and thus twenty or more licenses. Of the two transmission line "hot-stick" mounted toroidal or "shoebox" housed systems on the market, one offers only a radio link at 450 megahertz (MHz) and the other offers only a radio link at 926 MHz, both frequencies of which require a separate FCC license; no other data links, such as optical-through-air or fiber optic are offered in either system.
Though known available transformer systems provide accurate voltage measurement for metering or protective relaying, these existing state of the art PT and CT transformers are large and massive and thus expensive to buy, ship and install. Additionally, during installation or repair, they also require a line outage in which all adjacent equipment and the transmission lines they are to be connected to must be shut down. Installation further requires the use of an overhead crane to lift the heavy transformers and many skilled tradesmen to prepare structural concrete mounting pads and other equipment. Thus these PT and CT devices are extremely expensive to install on transmission lines. These systems are reliable over long periods in that they can last as much as forty years or so; however, their accuracy will drift over years of field use and there is no economically and technically feasible way to recalibrate them in the field. If a power company is metering $100,000,000 of power delivered on a line to a customer, and the metering system drifts off by one percent (1%) or more over a long period of field use, the errors can be in megadollars.
It is therefore, the object of the present invention to provide a power line monitoring system which is accurate, reliable and stable for installation and long term use on power transmission lines to monitor and measure line voltage, line current, line temperature, ambient temperature and phase angle. Phase angle is the angle in degrees between comparable points of the voltage and current waveforms. One degree is 1/360 of a single full alternating current (AC) cycle. Typically the time between, for example, the zero crossing of the negative going voltage sinusoid and the zero crossing of the negative going current sinusoid is measured. That time period is converted to degrees of phase angle phi (.phi.). The value (cosine .phi.) is the power factor. Power (P) equals the product of current (I) voltage (V) and power factor (cos .phi.) with current and voltage expressed in true RMS units. Thus: EQU P=IV cos .phi.
For .phi.=0 degrees, cos .phi.=1, and there are no reactive losses. If the current leads the voltage by 10 degrees, the power factor cos .phi.=cos (10.degree.)=0.985 and the load is capacitive. If the current lags the voltage by 10 degrees, the power factor cos .phi.=cos (10.degree.)=0.985 and the load is inductive.
It is yet another object of the present invention to provide a power line monitoring system which is compact and inexpensive enough to be easily mounted on transmission lines without interrupting line use.
Yet another object of the present invention is to be a power line monitoring system using an elegantly simple voltage divider technique to provide accurate independent metering of voltage.
Yet another object of the present invention is to provide a power line monitoring system which measures line voltage, line current, line phase angle, and line and ambient temperatures using electronic sensors in contact with the high voltage transmission lines.
Still another object of the present invention is to provide a power line transmission monitoring system with a measuring station mounted directly on the power line, directly communicating with a ground receiving and data processing station located nearby.
Still another object of the present invention is to provide a power transmission line monitoring system having a measuring station connected to the ground by a fiber optic cable transmitting linear (analog) and/or digital voltage, current, phase angle and temperature measurement data. The phase angle is derived from the voltage and current waveform data.
Yet another object of the present invention is to provide a power transmission line monitoring system using a simple voltage divider having a very large ohmic resistance incorporated into a fiber optic cable link forming a unitized resistive link/fiber optic cable assembly. Thus, the fiber optic cable assembly serves as both a data link to transmit data from the line to the ground, and also as a means of containing the voltage sensor voltage divider large ohmic resistance resistive link, which resistive link has a fixed connection to the line at line potential at one end and a fixed connection to the ground at ground potential at the other end across the voltage gradient.
Another object of the invention is to avoid the severe costs and operating complexities attendant with the massive high voltage to ground bridge structures of potential transformers and current transformers.
Yet another object of the present invention is to avoid the costs and operating complexities of present systems by measuring line voltage, current, phase angle, temperature and ambient temperature using electronic sensors employing components which are inexpensive and can be located within a single monitoring system measuring station located on and in contact with high voltage transmission lines.
Still another object of the present invention is to provide a live-line (no line outage) installation of a transmission line monitoring system which can easily be installed or removed using electric power industry manual "hot-stick" apparatus without interrupting the power line.
Still another object of the present invention is to provide a power line monitoring system which will eliminate the requirement for a dedicated potential transformer to measure current. This is accomplished by measuring line voltage, current, phase angle, temperature, and ambient temperature from a single solitary measuring station.
Still another object of the present invention is to process and transmit real time analog line voltage and current waveform data (and in some cases perhaps temperature data) to the ground using linear fiber optic or radio or optical-through-air transmitters and receivers with only group delay of typically only 20-50 nanoseconds in order to minimize delay time for purposes of fast system protection relaying and circuit breaking where the goal is to detect voltage and/or current faults as quickly as possible which means in some cases in less than 1 AC cycle of 16.7 milleseconds (ms).
Yet another object of the present invention is to provide a transmission line monitoring system which can transmit data to ground based receiving stations through a dual purpose cable incorporating a resistive link filament and optical fiber data links.
Yet another object of the present invention is to provide a power line transmission monitoring system which can digitize raw analog voltage, current, phase angle and line and ambient temperature data, either before or after data transmission from the power line measuring station to the ground based receiving and data processing station. Digital circuitry incorporated in the monitoring system may be dedicated to processing one parameter or may be shared by two or more parameters using multiplexing means depending upon how often parameter data is taken, digitized and transmitted. With multiplexing one data link can time-sequentially transmit multiple data parameters. Typically the data is digitized up in the power line measuring station. Typically the data digitized is true RMS line voltage, true RMS line current, phase angle, line and ambient temperature for purposes of metering and networking/load management.
Still another object of the present invention is to provide a transmission line monitoring system having a duplexing fiber optic cable link which allows simultaneous two-way flow of data, to and from the ground. Thus, a command and control computer on the ground can be connected to the fiber optic cable assembly to send and receive diagnostic and calibration data to and from the monitoring system measuring station connected to the high voltage power line phase conductor.
Yet another object of the present invention is to provide a transmission line monitoring system providing accurate measurement of voltage using a voltage divider technique and enhancing the accuracy of the voltage sensor by providing circuitry to correct for the effect of changes in the resistance of ohms of the voltage divider as a function of changes in ambient temperature and/or by employing a resistive link whose resistance has a very high tolerance and a very low temperature coefficient of resistance (TCR). In describing a resistor with, for example, a 0.1% or 0.01% tolerance, the words very high tolerance or precision tolerance or very close tolerance are all synonymous.